This invention concerns improvements in or relating to fabrics, in particular fabrics having physical characteristics suitable for arresting ballistic articles. Such articles typically include fragments of compressor fan blades from aircraft engines such as turbofans. However, the term `ballistic article` is also intended to embrace eg; bullets and shells, or fragments thereof.
It has for some years been common practice for manufacturers of aircraft engines having rotating parts to provide within the engine a barrier capable of arresting ballistic articles arising from mechanical failure within the engine. The object of this practice is to minimise the damage to the remainder of the engine that may be caused by such articles.
In the early days of aviation, such barriers were provided by rigid, metal components. However, these are of limited utility because of the tendency of metal barriers to transmit impulses directly to other parts of the engine, thereby causing potentially catastrophic damage.
The development of aramid fibres led to replacement of the rigid barriers by barriers comprised essentially of woven fabrics made of aramid yarns. Typically, the woven fabrics are produced in widths of up to 1000 mm that are wrapped several times about an annular frame defining eg the periphery of the compressor stage of a turbofan engine. The thus-wound fabric is effectively exposed, on the inner face of the barrier, to the exterior of the compressor stage, so that high velocity articles resulting from mechanical failure within the compressor stage tend to be thrown outwardly into the fabric wrap. The fabric absorbs the resulting impulse.
This method of arresting ballistic articles is successful because aramid fibres possess almost no elasticity yet are flexible and have extremely good tensile strength characteristics. Typically, the elongation to failure of an aramid fibre is less than 3%, yet the fibre can withstand huge tensile loads before such failure occurs. Thus, a woven fabric consisting of aramid fibres is most unlikely to rupture when it experiences the impulse from a ballistic article in an aircraft engine; yet the energy of such an impulse is successfully absorbed by the woven fabric structure without any significant part of the energy being transferred to the remainder of the engine components.
In this way, woven aramid fibre barriers have prevented many instances of catastrophic aircraft engine failure.
The high strength/low elasticity characteristics of aramid fibres also make them highly suitable as ballistic barriers in eg; flak jackets and bullet-proof vests.
In view of their characteristics, fabrics woven from aramid fibres are known as `rigid fabrics`. There are other fibres (including high-density polypropylene and polyethylene) that are also potentially suitable in such applications. The weaving of such alternative fibres also results in so-called rigid fabrics. The term "rigid fabric" also embraces fabrics made from mixes of fibres, not all of which need necessarily possess low elasticity/high strength characteristics.
Tests have revealed that in typical instances of aircraft engine component failure, known rigid fabric barriers exhibit extensions significantly greater than the approximately 3% figure mentioned above. The precise performance characteristics depend in part on the engine in which the fabric is installed.
There is a constant effort to improve the efficiency of aircraft engines, by reducing their specific fuel consumption characteristics. One way of achieving this is to increase the compressor fan area, thereby permitting a higher charge compression ratio to be used. However, for reasons of weight saving and because it is often not possible simply to increase the overall dimensions of an engine, such increases in fan area are usually accomplished at the expense of reducing the size of other components constituting the generally annular shape of the compressor chamber. Thus there is a need for a rigid fabric that offers comparable performance to previous rigid fabrics, whilst occupying a reduced volume and/or possessing reduced mass.
It is known from U.S. Pat. No. 4,699,567 to produce a ballistic barrier for an aircraft turbofan engine in the form of a fabric wrap comprising a plurality of squares of woven, rigid fabric. The squares are secured together in a series, by means of low strength stitching threads, for example cotton, to create an elongate fabric that is wrapped around the compressor stage of the turbofan engine during its construction.
The size of the squares is chosen so that when a length of the fabric is wrapped several times around the compressor stage, the joints between squares in the layers of fabric are out of phase with one another so that there are no radial lines of weakness in the fabric wrap.
The wrap is applied under low or zero tension. When a ballistic article such as a blade tip strikes the wrap, the joints between adjacent squares in the vicinity of the impact fail in a progressive and controlled manner, thereby absorbing the energy of the ballistic article. Thus the fabric of U.S. Pat. No. 4,699,567 damps the initially high frequency oscillation of the ballistic article in a short period.
However, the fabric wrap of U.S. Pat. No. 4,699,567 is complex and time consuming the manufacture, partly because of the need to produce numerous discrete squares of rigid fabric; and partly because of the need subsequently to stitch the squares together using a blanket stitch in yarn or low strength thread such as cotton. Such stitching has to be carried out as a separate step from the weaving of the squares.